1. Field of the Invention
The present invention relates to an address generating device for indexing a table in a distortion compensator of a wireless communication device, and to a method of generating the address.
2. Description of the Related Art
FIG. 1 to FIG. 4 explain problems in conventional techniques.
In a conventional distortion compensator, index addresses for a look up table (LUT) are generated in accordance with electric power supplied, and when the address is generated as the log of electric power supplied, the occurrence rate of the addresses generated in accordance with the electric power supplied presents distribution indicated as “through” in FIG. 1. This “through” appears in the case where LUT index addresses are linearly assigned with respect to the electric power supplied. Then, the occurrence distribution of addresses involves bias, specifically, in a range where the electric power is large the occurrence rate of addresses is low, which results in poor convergence in a distortion compensation table. Specifically, distortion compensation coefficients corresponding to addresses with a low occurrence rate are updated at a low frequency so that a value of the distortion compensation coefficient converges to a suitable value slowly. FIG. 2 shows an example of a value of a distortion compensation coefficient when index addresses with respect to electric power supplied are generated by a “through” method. In FIG. 2, the vertical axis represents the value of a distortion compensation coefficient after a prescribed time period, and the horizontal axis represents the value of an index address. As is obvious from this figure, some values of the distortion compensation coefficients are abnormal.
In view of the above problem, in the Patent Document 1, a method is disclosed in which the frequency of the occurrence of addresses is counted, and conversions of addresses are conducted such that the distribution of frequencies of occurrences of respective addresses is constant. However, when the distribution of addresses is constant, it sometimes happens that the number of addresses which can be assigned to the distortion compensation coefficient in a range with a large electric power is reduced so that distortion compensation characteristic deteriorates.
Patent Document 1
Japanese Patent Application Publication No. 2003-347944
FIG. 3 shows the relationship between input addresses (in proportion to electric power supplied) and output addresses (index addresses) in an address conversion unit for the case of the “through” method and the case of constant assignment. The line for the case of constant assignment represents an image of address conversion for the case where the occurrence frequency of index addresses is constant, and when the address frequency is constant, almost half of the input addresses are assigned to one address. In FIG. 3, output addresses are assigned to one value “19” when input addresses are greater than or equal to “11”. Accordingly, in the case of constant assignment, half of the input addresses which have a large electric power are compensated with one distortion compensation coefficient.
However, in a characteristic of an amplifier as shown in FIG. 4, the output power with respect to the electric power supplied to the amplifier distorts in a range where the electric power supplied is large. Thus, it is desired that the distortion compensation characteristic is precise in a range where the electric power supplied is large. However, in the case of constant assignment, in a range where the electric power supplied is large, distortion compensation is conducted with the smaller number of distortion compensation coefficients, accordingly, precise distortion compensation characteristics cannot be obtained.